The original objective of this project is to characterize by pharmacological and biochemical methods the adrenergic receptors in selected sympathetically innervated tissues in order to improve our understanding of factors at the receptor level which regulate the responses of these tissues to various adrenergic drugs and to sympathetic nerve stimulation. Experiments will be carried out (1) to ascertain whether the simultaneous occurrence of more than one type of adrenergic beta-receptor in a single cell type is a common phenomenon; (2) to elucidate the factors which control the variable ratio of beta1-to beta2-type receptors in selected smooth muscle cells; (3) to reexamine whether postjunctional adrenergic alpha-receptors in different sympathetic effectors in the same species are all of a single type. Characterization of receptors responsible for mediating a specific response in a given isolated tissue will be carried out by determining with appropriate pharmacological procedures affinities and efficacies of selected agonists, and affinities of selected competitive antagonists. In addition, we shall apply radioactive ligand-binding techniques for quantification of one or more types of beta-receptors or alpha-receptors in selected tissues. In addition to the original objective, a new major objective is to elucidate the mechanism by which acetylcholine relaxes blood vessels. We have already demonstrated with isolated vessels that this relaxation is the result of acetylcholine acting on the muscarinic receptors of endothelial cells, which then are stimulated to produce and release a substance that relaxes the smooth muscle cells. Pharmacological and biochemical studies will be directed at determining the nature of released relaxing substance and the mechanism whereby it activates relaxation.